Paramagnetic to antiferromagnetic phase transformation in sputter deposited Pt-Mn thin films

Sputter deposited, equiatomic Pt-Mn thin films used in giant magnetoresistive spin valves are found not to exist in the antiferromagnetic state required for device operation. Therefore, an annealing step is needed to induce a phase transformation from the as-deposited, paramagnetic A1 (fcc) phase to the antiferromagnetic L1(0) phase. The L1(0) phase is the thermodynamically stable configuration, but favorable kinetics for the transformation were only found above 260degreesC. The A1 to L1(0) phase transformation was studied by x-ray diffraction, transmission electron microscopy and differential scanning calorimetry (DSC). The nucleation and growth conditions were evaluated and an exothermic transformation enthalpy of -12.1 kJ/mol of atoms was determined. The kinetics of the reaction were simulated using the Johnson-Mehl-Avrami analysis, where the necessary parameters were determined by the Kissinger and Ozawa methods from constant scanning rate DSC experiments [H. Yinnon and D. R. Uhlmann, J. Non-Cryst. Solids 54, 253 (1983)]. The resulting simulations were compared to DSC data as well as isothermal x-ray peak shift data and a reasonable agreement was obtained. (C) 2003 American Institute of Physics.